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We report our very early optical observations of GRB 110530A and investigateits jet properties together with its X-ray afterglow data. A peculiar broadonset bump followed by a plateau is observed in its early R band afterglowlightcurve. The optical data in the other bands and the X-ray data are wellconsistent with the temporal feature of the R band lightcurve. Our jointspectral fits of the optical and X-ray data show that they are in the sameregime, with a photon index of $\sim 1.70$. The optical and X-ray afterglowlightcurves are well fitted with the standard external shock model byconsidering a delayed energy injection component. Based on our modelingresults, we find that the radiative efficiency of the GRB jet is $\sim 1\%$ andthe magnetization parameter of the afterglow jet is $<0.04$ with the derivedextremely low $\epsilon_B$ (the fraction of shock energy to magnetic field) of$(1.64\pm 0.25)\times 10^{-6}$. These results indicate that the jet may bematter dominated. Discussion on delayed energy injection from accretion of latefall-back material of its pre-supernova star is also presented.

GRB 110530A: Peculiar Broad Bump and Delayed Plateau in Early Optical Afterglows